Fractures of bones in the body often require internal fixation to obtain satisfactory position of the fractured elements for healing. As a general rule, compression of the fractured ends is desirable for promoting fracture healing. Many internal fixation devices are based on this philosophy to promote compression of the fractured elements.
Some situations occur, however, where it is more important to maintain one fragment in a specific position in space in order to preserve proper function of the extremity. In some fractures, this means holding the fracture element out to length and preventing shortening. In other situations, this means preventing a fracture element from rotating into an abnormal angular position.
One example of this type of situation occurs in the context of a distal radius fracture that has a radial styloid or radial column fragment. If the radial styloid is not held out to length, the bones of the carpus (wrist) will subside proximally and result in high deforming loads across the fractured articular surface. This can lead to loss of reduction of the joint surface and malposition of the wrist. In addition, since the adjacent ulna is of normal length and the radius becomes short, significant dysfunction can occur to the adjacent radioulnar joint. Finally, translation of the radial styloid to the radial side causes the end of the articular surface to spread apart, resulting in incongruity of the articular surface and a poor clinical result.
Since it is extremely important to maintain radial length, fixation of distal radius fractures should prevent collapse of the radial column. In addition, it is desirable to prevent radial translation of the radial side of the wrist by providing a buttress to the radial column.
Current implants do not adequately address this problem of maintaining length to the radial styloid. A conventional radial pin plate requires the implant to be placed entirely on the surface of both fracture fragments and requires a larger exposure to apply the implant to the surface of the radial styloid. In addition, since the pins that cross the radial styloid are thin, the fragment can slide along the pins and lose length.
Standard buttress fixation plates placed either dorsally or volarly also do not adequately address this problem. These also require exposure of the entire surface of the bone, which can be detrimental because of stripping of the blood supply and irritation of the soft tissues. This type of approach requires a bulky plate to be placed on the superficial surface of the distal fragment in a region where many tendons and ligaments are in close apposition to the bone; this can result in tendon irritation and even rupture. In addition, because these plates are placed on the dorsal or volar surface, the only means available for supporting the distal articular surface is limited to cylindrical posts or screws that are placed through holes in the plate. Although the screws or posts may be locked into the plate to prevent angulation in relation to the hole in the plate, they can only cross transversely across the distal bone fragment to exit along the opposite cortex, either dorsally or volarly depending on the placement of the plate or tines that extend perpendicularly to the plate. As such they can only act to buttress the subchondral bone at the apex of concavity of the articular surface at a single point. In addition, this buttress effect occurs only along the side of the screw or post. Since these posts or screws are designed to cross the bone either from dorsal to volar or from volar to dorsal, it is not geometrically possible to angle the tip of a post or screw into the apical corner of the radial styloid using a dorsal or volar plate.
Fixation screws or posts through volar, dorsal, or even a radial sided plate can only buttress a fragment by contact between the fragment and the side of the screw or post. If this occurs, the screw or post is placed under torque, increasing the forces in both the implant and bone/implant interface. This torque may lead to loosening and make the bone fragment more prone to slip off the fixation post. Increased stresses in the bone implant interface also increase the risk of failure.
Finally, because of the complex geometry and intimate apposition of tendons and ligaments against the volar and dorsal surfaces of the radius, trying to secure a distal radial styloid fragment using dorsal or volar fixation is prone to complications and is generally not used.
Standard hip plates also provide a similar situation in which the plate is fixed distally with an external side plate and screws, and has a second part that is placed internally within the femoral head to secure this fragment. However, these implants are intentionally designed to allow sliding and impaction of the femoral head to promote union; in this circumstance the loss of length is a minor problem compared to the possibility of non-union by maintaining length. In addition, with the existing hip screws and blade plates, no mechanism is present to buttress the subchondral bone of the femoral head with a smooth tip. These devices gain fixation to the metaphyseal bone only with threads (hip screws) or a blade (hip nails).